HP C Programmer's Guide (92434-90009)

14 Chapter2
Storage and Alignment Comparisons
Data Type Size and Alignments
Data Type Size and Alignments
This section discusses storage sizes and alignment modes for the HP 9000 and HP Apollo
systems as well as the VAX/VMS C, CCS/1000, and CCS/C 3000.
In all, there are a total of seven possible alignment modes which can be grouped into five
categories as described in Table on page 14.
NOTE With the exception of bit-fields, DOMAIN_WORD structure alignment is the same
as HPUX_WORD structure alignment,andDOMAIN_NATURAL structure alignment
is the same as HP_NATURAL structure alignment.
The alignment modes listed above can be controlled using the HP_ALIGN compiler pragma.
Table 2-1. The Alignment Modes
Alignment Mode Description
HPUX_WORD,
DOMAIN_WORD
HPUX_WORD is the native alignment for HP 9000 Series 300 and 400.
DOMAIN_WORD is the native alignment for HP Apollo Series 3000 and 4000.
The most restricted alignment boundary for a structure member is 2
bytes.
HPUX_NATURAL,
DOMAIN_NATURAL
HPUX_NATURAL is the native alignment for HP 9000 Series 700 and 800
and HP 3000 Series 900 and, therefore, is the default alignment mode.
DOMAIN_NATURAL is the native alignment for HP Apollo Series 10000. The
alignment of a structure member is related to its size (except for long
double and long pointers), and the most restricted alignment boundary is
8 bytes.
HPUX_NATURAL_S500 HPUX_NATURAL_S500 is the native alignment for HP 9000 Series 500. The
alignment of a structure member is related to its size, and the most
restricted alignment boundary is 4 bytes.
NATURAL NATURAL is an architecture-independent alignment mode for HP Series
300, 400, 700, and 800, and HP Apollo Series 3000, 4000, and 10000. In
the NATURAL mode, alignment of a structure member is related to its size,
the most restricted alignment boundary being 8 bytes. The difference
between HPUX_NATURAL and NATURAL are a 1-byte versus 2-byte minimum
structure alignment and size, and the bit-field rules. This alignment
mode is recommended when portability is an issue, since this mode
enables data to be shared among the greatest number of HP-UX and
Domain (HP Apollo) systems.
NOPADDING This mode does not arise from a particular architecture. The most
restricted alignment is 1 byte. NOPADDING alignment causes all structure
and union members and typedefs to be packed on a byte boundary, and
ensures that there will be no full byte padding inside the structure.
Bit-field members either are byte-aligned or aligned immediately
following a previous bit-field member, except in rare cases described in
the section "Alignments of Bit-Fields" below.